In a study published in the Journal of Neuroscience, researchers showed that improving mathematical performance by electrically stimulating certain parts of the brain led to impairments in others regions — and vice versa.

The scientists, led by Roi Cohen Kadosh of Oxford University, used a technique called transcranial electric stimulation (TES), a non-invasive procedure that previous studies linked to improved cognitive skills when applied to specific brain regions. Electrodes placed on the scalp use low levels of electric current to stimulate nearby brain areas, triggering a slight tingling, but not painful, sensation.

Nineteen young adults, divided into three groups, participated in the study. One group received 20 minutes of stimulation on both sides of the head over the posterior parietal cortex (PPC), a brain area critical for processing numbers. A second group was stimulated similarly over the dorsolateral prefrontal cortex (DLPFC), which is involved in learning more generally. The third group received fake stimulation: they felt the same tingling sensation as the real thing, but experience no significant brain effects.

Following the stimulation on each day, over the course of six days, the participants were trained for two hours a day to recognize symbols that the researchers assigned to stand for specific numbers, similar to learning numerals in a different language. First, they were tested on how fast they could identify which of the two new symbols stood for a larger amount. Later, they performed the same task, but with a twist: one of the symbols was presented in a larger font than the other, but did not necessarily always represent the larger amount. The volunteers had to identify either the larger or smaller sized symbol while ignoring the numerical meaning of its amount that they had just learned.

While it seems that would be easy, and the participants could ignore the numerical value of the symbols while focusing only on their physical size, those values the participants had just spent hours attaching to the symbols interfered with this seemingly simple task. These two tests represented two different aspects of learning: the first involved simply recognizing and memorizing the meaning of a mathematical symbol, while the second helped to measure to what degree that knowledge had become “automatic.” When there is a clash between a meaning you know well and its physical representation— like the smaller valued-number represented in a larger font — it takes longer to identify the smaller number because you know its value contrasts with its physical font size. This slowing of reaction time shows that the understanding of the symbol has become somewhat rote.

The researchers found that stimulating the PPC helped participants to learn the meaning of the symbols faster, compared to those who received sham stimulation. But it also impaired their ability to internalize the symbols and react automatically to them, suggesting that it might affect the ability to use new information rapidly and without conscious effort. Stimulating the DLPFC had the opposite effect: it impaired initial learning but enhanced automatic learning. And these effects were specific to the new material: they didn’t affect performance on either task with ordinary numbers.

“The current results clearly demonstrate that the enhancement of a specific cognitive ability can happen at the expense of another ability,” the authors write. The brain may be shifting resources in response to the stimulation, although it’s not clear how this compensation works. It is also possible that some types of stimulation might not have these compensatory, negative side effects on other brain activities. But the results highlight how inter-related brain regions and functions are; previous research on improving learning, for example, focused primarily on enhancing brain regions and looking for physical side effects, rather than exploring potential cognitive trade-offs.

Such a balancing of effects isn’t surprising, since the brain works hard to keep its chemical and electrical systems in a limited range, pushing back in one way or another if certain processes or levels exceed a certain range. These feedback loops mean that there are few benefits that have no costs and this research suggests that any attempts at cognitive enhancement — whether with drugs or electric stimulation or other methods— should be studied carefully for their potential harms as well as their promise.